The Sims 4 performing well, Maxis producer Grant Rodiek said on the official Sims forum (cache here).
"We're not working on Sims 5," Rodiek said. "We're not thinking about Sims 5. If Sims 4 isn't successful, there won't be a Sims 5." This isn't an absolute credo, however – when another fan asked for clarification on that statement, Rodiek said, "It's not all or nothing, per se, but something has to pay for 5."
The Sims 4 topped the UK charts last week. We've been playing it as an MTV's Real World simulator, with members of the Joystiq staff living together, cooking for each other, working and building rocket ships.